If the old time scooter and moped engine manufacturers would jump on the bike engine bandwagon, we'd have a lot better choices. A guy on Youtube put an Italian Morini moped engine on a bike after concluding the new Chinese kit he bought was junk. Puch, Sachs, Benelli and others could really turn out some high quality, dependable engine kits and increase their worldwide market. But, even though they don't make kits, you can still mount one of their engines and use the Chinese sprocket kit and gas tank.
YouTube - Motorized bicycle with Italian moped engine